To ensure a desired communication quality in a wireless communication system, power and rate control are often deployed. One type of power and rate control aims to ensure that a certain fixed rate is achieved for each user by adjusting the Carrier-to-Interference-Noise-Ratio (CINR) to a per user CINR target (or to an equal CINR target for all users). This approach is denoted CINR balancing (sometimes the N is dropped and it is referred to as CIR balancing). Note that the term user will interchangeably be used with the terms link and TX-RX pair.
A number of distributed CINR balancing schemes has been developed. In S. A. Grandhi, R. Vijayan, and D. J. Goodman, “Distributed power control in cellular radio systems,” IEEE Trans. Commun., pt. 1, vol. 42, no. 2-4, pp. 226-228, 1994, the Distributed Power Control (DPC) algorithm was introduced. Based on the mth iteration power value Pu(m), CINR Γu(m) and target CINR ΓuTarget for a link u, a new power value for link u may be calculated as
                              P          u                      (                          m              +              1                        )                          =                              P            u                          (              m              )                                ⁢                                    Γ              u              Target                                      Γ              u                              (                m                )                                                                        (        1        )            An intuitive rational for the form of (1) is by noticing that for the CINR, if all interference is kept constant, the CINR is linear in the transmit power of the own link. Hence, if it is desired to increase the CINR with some factor, the power need to be increased with the ratio between the desired CINR and the current CINR.
The CINR balancing idea was developed for voice services in narrowband systems.
However, today's systems are often of broadband type. In order to handle the wider bandwidth, the wider band is typically divided into multiple smaller bands on which information is transferred over. A typical scenario where this applies is OFDMA (Orthogonal Frequency Division Multiplex access).
Another scenario could be the use of multiple bands, possible even residing in widely distant parts of the spectrum, in cognitive radio systems.
While CINR balancing could be used on each spectrum resource, such as a subcarrier, it does not account for or exploit that different spectrum resources often fades independently and that it would be wiser to reallocate power to subcarriers with a good gain to noise ratio rather than poor ones.
In the case of cognitive radio (or any other system) using bands on significantly different frequency bands, the mean path loss and interference situation of each band may also differ significantly, and it may make sense to reallocate power to bands where it best pays off, e.g., in terms of data rate.